Treatment of leukemic guinea pigs with 2 courses of cytoxan over a 2 week period resulted in suppression of the disease for approximately 5 weeks before relapse and death of 95% of drug treated animals. Simultaneous administration of C. parvum and leukemic blast cells following cytoxan therapy resulted in 50% long term survivors at 100 days as compared to no survivors in the drug treated group. Blast cells, C. parvum, BCG and pyran copolymer given alone during the remission period were found ineffective. Surgical removal of a primary chemically-induced sarcoma in guinea pigs followed by cytoxan 4 days later resulted in 90% animals dying from metastasis to lungs and mesenteric nodes. Immunotherapy is being incorporated into the above modality of therapy in hopes of controlling metastatic lesions. The presence of RNA-directed polymerase activity and RNA observed in molecular hybridization studies ascribes to one type of virus which exhibits a variety of morphological states in electron micrographs involving L2C guinea pig leukemia. An enhanced incidence of spontaneous mammary carcinomas in C3H/HEN mice was observed following ovarian ligation. The enhancement is due to either a hormonal effect of ligation or presence of a "blocking factor" interfering with cell mediated response.